There is a growing demand for location awareness in short range radio networks, particularly in ultra wideband (UWB) networks. Typically, the location of a node in the network is determined based radio ranging measurements.
UWB or digital pulse wireless communication is a wireless technology for transmitting large amounts of data over a wide spectrum of frequency bands with very low power and for a short distance. UWB radio signals not only can carry a huge amount of data over a short distance at very low power, e.g., less than 0.5 milliwatts, but have the ability to carry signals through doors and other obstacles that tend to reflect signals having more limited bandwidths and a higher power.
UWB signals are transmitted as digital pulses that are timed very precisely on a carrier signal across a very wide spectrum of frequencies. A transmitter and a receiver are synchronized to send and receive pulses with an accuracy of trillionths of a second. On any particular frequency, the UWB signal has less power than normal and anticipated background noise. Theoretically, interference with conventional radio signals is negligible.
UWB communication has three main types of application. In radar applications, the UWB signal penetrates nearby surfaces but is reflected by surfaces that are farther away, allowing objects to be detected behind walls or other coverings. In data transmission applications, digital pulses allow a very low powered and relatively low cost signal to carry information at very high data rates over a short range. In location awareness applications, ultra wideband digital pulses allow accurate ranging estimate between different devices.
UWB applications communicate in accordance with a protocol stack that includes a physical layer (PHY), a media access control (MAC) layer, a network layer, a transport layer, a session layer, a presentation layer, and an application layer.
UWB two-way ranging is performed by two transceivers. Conventionally, a range packet is sent from a device A to a device B. Upon receipt of the range packet at the device B, the range packet is returned to device A. Measuring the length of time required for this roundtrip can reveal the distance between the two transceivers.
For example, a transmitter can send a signal to a receiver at t1. The receiver, as soon as possible, returns a reply signal to the transmitter. The transmitter measures the time of arrival (TOA) of the reply signal at time t2. An estimate of the distance between the transmitter and the receiver is the time for the signal to make the round trip divided by two and multiplying by the speed of light is, i.e.:
  D  =                                                  t            1                    -                      t            2                                      2        ⁢          c      .      
To meet the need for improved and private location awareness in UWB, an IEEE 802.15.4a Task Group (TG) has been established to develop a UWB-based physical (PHY) layer standard with a precision ranging capability. An UWB signal has a relative bandwidth larger than 20%, or an absolute bandwidth of at least 500 MHz. One type of an UWB system is an impulse radio (IR). IR uses extremely short duration pulses to generate signal waveforms, and allows fine time resolution of channel multipath characteristics, which is important in identifying the line of sight signal for precision ranging.
Various publications have described ways to accurately estimate the distance between two devices. In a paper by J-Y. Lee and R. A. Scholtz, “Ranging in a dense multipath environment using an UWB radio link,” IEEE Trans. Select Areas in Communications, vol. 20, issue 9, pp. 1677-1683, Dec. 2002, the entire contents of which is incorporated by reference, a time-of-arrival (TOA)-based ranging scheme using an ultra-wideband (UWB) radio link is described. That ranging scheme implements a search process for the detection of a direct path signal in the presence of dense multipath, utilizing generalized maximum-likelihood (GML) estimation. Models for critical parameters in the process are based on statistical analysis of propagation data. The process is tested on another independent set of propagation measurements. That UWB ranging system uses a correlator and a parallel sampler with a high-speed measurement capability in the transceiver to accomplish two-way ranging in the absence of synchronized clocks. In a paper by S. Gezici, Z. Tian, G. B. Giannakis, H. Kobayashi, A. M. Molisch, H. V Poor, Z. Sahinoglu, “Localization Via UWB Radios,” IEEE Signal Pro. Magazine, v. 22, n. 4, pp. 70-84, Jul. 2005, the entire contents of which is incorporated by reference, localization techniques relying on wireless ultra-wideband (UWB) signaling are described. Various localization alternatives are considered and the UWB time-of-arrival based one is found to have a highest ranging accuracy.
A further important step is to derive the position (location) of a node (device) A from the estimates of the ranges between this device A and other nodes. Using three or more such range estimates, the position (relative to the other nodes) can be determined. If the ranges are known ideally, then the position estimate also is perfect, and it does not matter whether three or more range estimates are present. Additional range estimates, e.g., more than three, just confirm the position estimate. However, in practice, the accuracy of the range estimate is always limited. In that case, a larger number of range estimates helps to decrease the error in the position estimate. Different combinations of range estimates result in different position estimates, and combining those different position estimates improves the overall accuracy. When using that technique, it is important to know the reliability of the different range estimates, and this reliability has to be communicated through the network to the nodes that make the actual position estimates.
Communicating the reliability of range estimates in an efficient way is thus important, but nontrivial. Ideally, the probability density function (pdf) of the range estimate should be communicated. However, in order to reduce the overhead, limited information can be transmitted over the network and the transmission occurs digitally. Therefore, quantization has to occur.
A conventional way for quantizing pdfs is to express the pdfs in parametric form, and communicate the suitably quantized parameters through the network. A simple example of that is a description of a Gaussian pdf, where only the mean and the variance has to be signaled. However, no parametric form of the range estimate pdf is known; it is only established that the pdf is not Gaussian. Therefore the parametric representation cannot easily be applied to ranging data.
Therefore, the current state of the art defines nominal intervals describing the accuracy of the estimate (henceforth called confidence intervals) and signals the level of confidence into each of them. For example, a proposal from TimeDomain Corporation for the IEEE 802.15.4a standard defines a 5-bit range quality indication, see Vern Brethour, “Ranging Values,” IEEE 802.15-05-0679-01-004a, incorporated herein by reference. Two bits are used for a confidence interval, and three bits are used for a confidence level. The possible range resolution is very small, because only two bits are used to indicate the confidence interval.
The requirements for range accuracy can vary widely, depending on the applications. For line-of-sight situations with high transmission bandwidth, e.g., 7.5 GHz, range accuracies of less than 1 cm are desired. For other situations, e.g., non-LOS, non-coherent receivers, and distances between nodes larger than 10 meters, range accuracies of more than 1 meter are desired. Therefore, the traditional method requires the definition and signaling of a large number of ranges, which in turns requires the transmission of a large number of bits.